Compared to second-generation mobile communication systems, one of the most important aspects of third-generation mobile systems is enhanced packet-data access. The ITU standard known as wideband code division multiple access (WCDMA), Release 99, provides for data rates of 384 kbit/s for wide area coverage and up to 2 Mbit/s for hot-spot areas, which is sufficient for most existing packet-data applications. However, as the use of packet data services increases and new services are introduced, greater capacity will be required. WCDMA Release 5 extends the specification with, among other things, a new downlink transport channel that enhances support for interactive and background services, and, to some extent, streaming services, yielding a considerable increase in capacity compared to Release 99. Release 5 also significantly reduces delay and provides peak data rates of up to 14 Mbit/s. This enhancement, which commonly goes under the abbreviation, HSDPA (high speed downlink packet access), is the first step in evolving WCDMA to provide even more outstanding performance.
An important objective of the HSDPA design has been to retain the functional split introduced in Release 99 between layers and nodes. Minimal architectural changes should ensure a smooth upgrade and enable operation in environments where not every cell of the network supports the new functionality. Nonetheless, given that the key features are rapid adaptation to changes in the radio environment and fast retransmission of data, it follows that the corresponding functionality should be placed as close to the air interface as possible. The introduction of HSDPA therefore affects primarily the radio base station (RBS, also called Node B), in particular through the addition of a new medium access control sub-layer (MAC-hs). The architecture retains the radio network controller (RNC) functionality of Release 99. By switching channels in the RNC, the system can easily handle terminal movement from a cell that supports HSDPA to one that does not. That is, when switching a terminal from the high-speed downlink shared channel (HS-DSCH) to a dedicated channel (DCH) in a non-enhanced cell, the system ensures uninterrupted service, albeit at a lower data rate. Conversely, when a terminal enters a cell that supports HSDPA, the system can switch the terminal from a dedicated channel to the HS-DSCH. FIG. 1 illustrates schematically a WCDMA system with MAC-hs functionality.
The scheduler, which is part of the MAC-hs in the Node B, is a key element that determines the overall behavior of the system. For each transmission time interval (TTI) on the HS-DSCH, the scheduler determines which terminal (or terminals) the HS-DSCH should be transmitted to, and, in collaboration with the link adaptation mechanism, at what data rate. A significant increase in capacity can be obtained if, instead of allocating radio resources sequentially (i.e. so-called round-robin scheduling), the scheduler employs channel dependent scheduling: that is, the scheduler prioritises transmissions to terminals having favourable instantaneous channel conditions. By prioritizing these terminals, the network experiences mostly good conditions. The effect is greater diversity at the system level, hence the term “multi-user diversity”. As load in the cell increases, the number of terminals queued for scheduling increases. This in turn raises the probability of being able to schedule transmissions to terminals with good channel quality.
In considering and comparing scheduling algorithms, it is necessary to distinguish between two kinds of variations in service quality:                rapid variations in service quality; and        long-term variations in service quality.        
Rapid variations in service quality are due, for example, to multipath fading and variations in the interference level. For many packet-data applications, relatively large short-term variations in service quality are acceptable or go unnoticed. Long-term variations in service quality are due, for example, to changes in the distance between the terminal and the Node B. Such long-term variations should generally be minimised.
A practical scheduling strategy exploits the short-term variations while maintaining some degree of long-term fairness between users. In principle, system throughput decreases the more fairness is enforced. Therefore, a trade-off must be reached. Typically, the higher the system load, the greater the discrepancies between different scheduling strategies. Channel-dependent schedulers must estimate the instantaneous radio conditions of the terminal. Therefore, each terminal that uses high-speed services transmits regular channel quality reports to the Node B via the HS-DPCCH (high speed dedicated physical control channel, an uplink control channel). The scheduler might also use other information available in the Node B to assess terminal radio conditions.
HSDPA includes a new frame for transporting MAC-d PDUs from the RNC to the Node B. The UTRAN user-plane protocol termination points of HSDPA are illustrated in FIG. 2. The user-plane “flow” through the different layers (of FIG. 2) is illustrated in FIG. 3. RLC SDU's (, i.e. higher layer data units—typically IP packets) are segmented into RLC PDUs—RLC (and possibly MAC-d) headers are added in the RNC. The RLC/MAC-d PDUs are carried in Iub user-plane frames (not illustrated in the figure) to the Node B, where the MAC-d PDUs (MAC-hs SDUs) are queued in MAC-hs buffers (one or more buffers or queues are provided for each user terminal) for scheduled transmission over the air. [The Transport Network Layer (TNL) can be realised either with ATM or IP.]
Depending on the link quality and the amount of scheduled resources, different numbers of MAC-hs SDUs fit into a given transport block. The HSDPA scheduler in the Node B can be designed with many different objectives in mind. For example, a round-robin scheduler assigns equal resources to all active users without taking into account any differences in link quality, while a Maximum C/I scheduler always chooses the user with the best link quality. A Maximum-C/I scheduler delivers maximum cell throughput, but this scheduler can be very unfair to badly placed users in a loaded cell. Typically, some combination of the two extremes above, termed a “Proportional Fair” scheduler, is used. Typical inputs to the HSDPA scheduler include the MAC-hs buffer fill levels in the Node B and the CQI (link quality) reports from the user terminals.